Field of the Invention
The present invention relates to a first composition (A) comprising (i) at least one compound (I) selected from the group consisting of triallyl isocyanurate, triallyl cyanurate, wherein the compound (I) is preferably triallyl isocyanurate, and (ii) at least one bis(alkenylamide) compound. In addition, the present invention also relates to a second composition (B) comprising the first composition (A) and at least one polyolefin copolymer. Finally, the present invention relates to the use of the composition (B) for production of a film for encapsulation of an electronic device, especially a solar cell.
Discussion of the Background
Photovoltaic modules (photovoltaic=“PV”) typically consist of a layer of symmetrically arranged silicon cells welded into two layers of a protective film. This protective film is itself stabilized in turn by a “backsheet” on its reverse side and a “frontsheet” on its front side. The backsheet and frontsheet may either be suitable polymer films or consist of glass. The function of the encapsulation material is essentially to protect the PV module from weathering effects and mechanical stress, and for that reason the mechanical stability of the particular encapsulation material is an important property. In addition, good encapsulation materials have a rapid curing rate, high gel content, high transmission, low tendency to temperature- and heat-induced discolouration and high adhesion (i.e. a low tendency to UV-induced delamination).
The encapsulation materials described for this purpose in the related art (for example WO 2008/036708 A2) are typically based on materials such as silicone resins, polyvinyl butyral resins, ionomers, polyolefin films or ethylene-vinyl acetate copolymers (“EVA”).
Processes for producing such encapsulation films are familiar to those skilled in the art (EP 1 164 167 A1). In these processes the crosslinkers, together with a polyolefin copolymer (and possibly further additives), are homogeneously mixed in an extruder for example, and then extruded to give a film. The process described in EP 1 164 167 A1 relates to encapsulation films based on EVA but is also applicable to films made of other materials, for example those mentioned hereinabove.
The encapsulation of the silicon cells is typically performed in a vacuum lamination oven (EP 2 457 728 A1). To this end, the layer structure of the PV module is prepared and initially heated up gradually in a lamination oven (consisting of two chambers separated by a membrane). This softens the polyolefin copolymer (for example EVA). At the same time, the oven is evacuated in order to remove the air between the layers. This step is the most critical and takes between 4 and 6 minutes. Subsequently, the vacuum is broken via the second chamber, and the layers of the module are welded to one another by application of pressure. Heating is simultaneously continued up to the crosslinking temperature, the crosslinking of the film then taking place in this final step.
The use of EVA in particular is standard in the production of encapsulation films for solar modules. However, EVA also has a lower specific electrical resistance ρ than polyolefin films for example. This makes the use of EVA films as encapsulation material less attractive, since it is specifically encapsulation materials having high specific electrical resistance ρ that are desired.
This is because what is called the “PID” effect (PID=potential-induced degradation) is currently a major quality problem for PV modules. The term “PID” is understood to mean a voltage-induced performance degradation caused by what are called “leakage currents” within the PV module.
The damaging leakage currents are caused not only by the structure of the solar cell but also by the voltage level of the individual PV modules with respect to the earth potential—in most unearthed PV systems, the PV modules are subjected to a positive or negative voltage. PID usually occurs at a negative voltage relative to earth potential and is accelerated by high system voltages, high temperatures and high air humidity. As a result, sodium ions migrate out of the cover glass of the PV module to the interface of the solar cell and cause damage (“shunts”) there, which can lead to performance losses or even to the total loss of the PV module.
The risk of occurrence of a PID effect can be distinctly reduced by increasing the specific electrical resistance ρ of the encapsulation films.
The specific electrical resistance ρ or else volume resistivity (also abbreviated hereinafter to “VR”) is a temperature-dependent material constant. It is utilized to calculate the electrical resistivity of a homogeneous electrical conductor. Specific electrical resistance is determined in accordance with the invention by means of ASTM-D257.
The higher the specific electrical resistance ρ of a material, the less photovoltaic modules are prone to the PID effect. A significant positive effect in increasing the specific electrical resistance ρ of encapsulation films is therefore the increase in the lifetime and efficiency of PV modules.
The related art discusses the problem of the PID effect in connection with encapsulation films for PV modules in CN 103525321 A. This document describes an EVA-based film for encapsulating solar cells, which comprises triallyl isocyanurate (“TAIC”) and trimethylolpropane trimethacrylate (“TMPTMA”) as co-crosslinkers and, as further additives, preferably comprises a polyolefin ionomer and a polysiloxane for hydrophobization. This film exhibits a reduced PID effect. However this film has the disadvantage that polyolefin ionomers are relatively costly. Moreover, polysiloxanes have an adverse effect on adhesion properties. In addition, the examples do not give any specific information as to what improvements are achievable with what concentrations.
A crosslinker combination of TAIC and TMPTMA is also described by JP 2007-281135 A. The TMPTMA here brings about acceleration of the crosslinking reaction and hence leads to elevated productivity.
JP 2012-067174 A and JP 2012-087260 A describe an encapsulation film for solar cells based on EVA/a polyolefin which comprises not only TAIC but also, for example, ethylene glycol dimethacrylate, neopentyl glycol dimethacrylate, hexane-1,6-diol dimethacrylate as crosslinkers. These co-crosslinkers initially retard the crosslinking reaction somewhat and thus increase the processing time window.
JP 2009-135200 A likewise describes crosslinkers comprising TAIC and various (meth)acrylate derivatives of polyfunctional alcohols, and what is described in this case is improved heat resistance combined with a lower tendency to delamination of the EVA-based encapsulation.
JP 2007-281135 A and JP 2007-305634 A describe crosslinker combinations of TAIC and trimethylolpropane triacrylate (“TMPTA”) for use in the production of multilayer co-extruded EVA encapsulation films for solar cells.
Similar combinations of crosslinkers for solar cell encapsulation films are described, for example, by JP 2013-138094 A, JPH11-20094, JPH11-20095, JPH11-20096, JPH11-20097, JPH11-20098, JPH11-21541, CN 102391568 A, CN 102504715 A, CN 102863918 A, CN 102911612 A, CN 103045105 A, CN 103755876 A, CN 103804774 A, US 2011/0160383 A1, WO 2014/129573 A1.
There is accordingly a need for novel co-crosslinker systems, especially for production of encapsulation films for solar cells, which result in a markedly increased electrical resistance compared to films crosslinked in accordance with the prior art, in order thus to lead to a reduction in the PID risk when used in photovoltaic modules.